1. Field of the Invention
The invention generally relates to radio frequency antennas, and in particular, to omnidirectional antennas.
2. Description of the Related Art
In certain situations, an antenna with an omnidirectional pattern is desirable. For instance, such a characteristic is typically preferred for an antenna in a transmitter application, such as a wireless access point. In other situations, an omnidirectional pattern may be required by a regulation, such as an FCC regulation. In other situations, antenna having a relatively good axial ratio characteristics for circularly polarized waves is desired.
One example of a conventional omnidirectional antenna is known as a turnstile antenna. Such an antenna is constructed from four quarter wavelength arms, and each arm is energized with 90 degree phase intervals between each arm. 0 and 180 degrees of phase shift are available from the center core (or center conductor) and the shield (or outer conductor), respectively, of a coaxial cable. For 90 and 270 degrees, typically, a quarter wavelength phase shift is implemented with a length of cable a quarter wavelength long. See, for example, U.S. Pat. No. 2,086,976 to Brown. Other phase shifting circuits can also be used. See, for example, U.S. Pat. No. 3,725,943 to Spanos.
Another example of a conventional omnidirectional antenna is known as a cross-dipole antenna. A cross-dipole antenna is driven by a single coaxial cable and is advantageously compact. In addition, one pair of arms (first dipole) is longer than a second pair of arms (second dipole) such that in an ideal case, phase shifts of 45, 135, 225, 315 degrees are established by the arms themselves without a need for an external phase shifter or a second coax. See, for example, U.S. Pat. No. 2,420,967 to Moore; the background discussion (FIG. 7) within U.S. Pat. No. 6,163,306 to Nakamura, et al.; Japanese Patent Application Publication No. H04-291806 by Kazama; and the background discussion (FIG. 10B) within U.S. Pat. No. 6,271,800 to Nakamura, et al.
However, Applicant has observed that conventional omnidirectional antennas undesirably exhibit null patterns, which can cause an antenna or a system to fail a specification, reduce yield, or otherwise incur costly tuning procedures.
FIG. 1 illustrates an antenna pattern 102 that results when the arms of the cross-dipole antenna are driven by currents of unequal amplitudes. FIG. 2 illustrates an antenna pattern 202 that results when the arms of the cross-dipole antenna are not driven with precise 90 degree phase shifts, that is, are not in quadrature. Each of the patterns illustrated in FIGS. 1 and 2 is easily correctable by one of ordinary skill in the art, as the source of the problem was recognized.
FIG. 3 illustrates a top-view of a prior art cross-dipole antenna. See, for example, U.S. Pat. No. 2,420,967 to Moore. A coaxial structure, such as a coaxial cable feedline, connector, bracket, adapter, frame, or the like, includes a center conductor 302 and an outer shield 304. In a coaxial cable, a dielectric material fills the space between the center conductor 302 and the outer shield 304.
In counterclockwise order from above, the antenna has a first arm 312, a second arm 314, a third arm 316, and a fourth arm 318. A mirror image of the antenna is also applicable. In the conventional cross-dipole antenna, the first arm 312 and the third arm 316 share the same length (as measured from the center of the coaxial structure). The second arm 314 and the fourth arm 318 share the same length.
FIG. 4 illustrates an example of an antenna pattern for a cross-dipole antenna according to the prior art that can be encountered when the diameter of the outer conductor (shield) of a coaxial cable is not negligible with respect to wavelength. The antenna pattern can vary substantially from that of a desired omnidirectional pattern. The pattern 402 illustrated in FIG. 4 is based on a simulation as will be discussed later in connection with FIG. 8. The antenna phasors 404 are not of equal magnitude and are offset from a quadrature orientation (90 degrees). Applicant is not aware of conventional techniques in the art for correcting the asymmetric antenna pattern illustrated in FIG. 4 that is encountered with cross-dipole antennas.